We are sequencing the four proteins found in the bacteriophage phi X 174. This phage is a moderately complex phage appearing in the electron microscope as a regular icosahedral particle with projections or spikes at each vertex. The particle contains four virus coded proteins; one forming the icosahedral capsid, two forming the spikes and the fourth protein located either within the spike or attached to the viral DNA. This phage has well characterized genetics with a genome of seven cistrons estimated to comprise almost all of the coding potential of the phage DNA. It is easy to select and characterize mutants with this phage. Many enzymes have structural as well as functional constraints. An aim of this project is to obtain data to aid in the understanding of how amino acid sequences of such proteins specify such restraints. A functional structure such as a virus built up from various protein subunits offers a model system in which structural alterations in the protein may be easily assayed. Our approach will be to determine the amino acid replacements in the phi X coat proteins which lead to various classes of mutant phenotypes. Thermal stability variants such as temperature sensitive and cold sensitive mutants will be examined. Amino acid replacements giving rise to proteins with altered binding properties within the phage structure will also be characterized.